Reduced volume heat exchangers

ABSTRACT

A heat exchanger for delivery of heat transfer fluid to a process heat transfer surface is provided, the heat exchanger is in contact with a process fluid and the heat transfer surface comprises at least five heat transfer conduits each having a cross sectional area for the flow path of less than 2000 square millimetres wherein the linear velocity of the heat transfer fluid through the heat transfer conduits is from 0.5 to 20 m·s −1  and adapted so that the temperature of the heat transfer fluid changes by at least 1° C. when the system is operating at design load, the exchanger enables more accurate temperature measurement and control in physical and chemical reactions. Also provided is a heat transfer system in which the heat transfer conduit for passage of the heat transfer fluid is attached to an expansion plate which is in contact with the heat transfer surface and enables independent movement of the heat transfer conduit and the heat transfer surface as their temperature change due to changes in temperature and/or pressure.

The present invention relates to improvements in or relating to heatexchangers.

Heat exchangers are used to add or remove heat from gases liquids andsolids. Different designs and applications exist but the presentinvention is concerned with types where heating or cooling fluid flowswithin a conduit which is within or in close proximity to the materialbeing heated or cooled. Of particular interest however are those wherethe heat transfer fluid flows through some form of jacket, coil or pipe.Examples of this include tanks or pipes with internal or externaljackets, coils or plates. The present invention is also applicable tomany types of equipment designed for specific process functions such asreactors, reaction calorimeters, fermenters, cell growth vessels,extruders, dryers, mixers, mills, filters etc.

The heat exchangers with which the present invention is concerned aredesigned to have reduced inventories of heat transfer fluid within thejacket, coil or plate. They are referred to in this document as reducedvolume heat exchangers.

The gas or liquid which is used to deliver and remove heat is referredto as the heat transfer fluid. The heat transfer fluid may be aproprietary product such as Syltherm XLT, Dowtherm J or a nonproprietary fluid such as water, ethylene glycol or any other fluidwhich is suitable as a heat transfer fluid. The heat transfer fluidmight also be a gas or vapour which can condense or boil.

The material which is to be heated or cooled is referred to as theprocess material. The process material may be liquid, gas, powder,solids or mixtures thereof. The surface which separates the heattransfer fluid or heat transfer fluid conduit from the process materialand, that part of the surface which is in direct contact with theprocess material is referred to as the process heat transfer surface.The volume of heat transfer fluid contained within the conduit may bereferred to as hold up volume. Where there is a containment conduit forheat transfer fluid which is not part of the heat transfer surface, thisis referred to as a conductor pipe.

Conventional jackets heat exchangers employ (as illustrated in FIG. 1),external half coils (such as those illustrated in FIGS. 2 & 4) andinternal coils (as shown in FIG. 3). These heat exchanger systems aredesigned on the principle that heat transfer fluid turbulence should bemaximised for good distribution and transmission of heat. In order toachieve this, excess volumes of heat transfer fluid are passed throughthe heat exchanger.

The traditional design approach for heat exchangers has been to exposethe process material to a surface behind which is a large bath or largeplugs of turbulent heat transfer fluid. The present invention on theother hand is based on using multiple thin films or thin tubes of heattransfer fluid. According to the invention the flow paths of the heattransfer fluid are relatively short and the residence time is alsoshort.

The concept of the present invention described herein has superfinitesimilarities to bolt on or clamp on jackets, sometimes referred to as“limpet jackets”. The key difference of the present invention is thatwhilst the limpet jackets deliver compromised heat transfercapabilities, the design of the present invention delivers substantiallyenhanced heat transfer capabilities.

The present invention which embodies the reduced volume concept enablesdifferent design principles to be employed. A greater number of smallerheat transfer conduits are used for delivering heat transfer fluid tothe process heat transfer surface (or directly to the process materialin the case of internal pipes, coils or plates). We have found thatconsiderable benefits can be realised using these techniques.

We have found that the reduced volume design usually (but not always)contains conduits for the heat transfer fluid rather than half coils orjackets. In addition the use of external jackets or conduits providesfor the designer, the freedom to pick a material for containment of theheat transfer fluid. This has the benefit that a fully contained pipe orconduit does not have to be welded to the surface of the heat exchangerto maintain good containment integrity (of the heat transfer fluid) anda variety of attachment techniques such as glue, solder, clamps orguides can be used. Thus a material like copper having higher thermalconductivity can be used to deliver heat to the heat transfer surfaceirrespective of the process material. Because the conduits (carrying theheat transfer fluid) are small and made of a conductive material, heatcan be usefully transmitted from the heat transfer fluid over the fullwetted perimeter of the conduit. Good heat transfer at the process heattransfer surface can therefore be achieved by even distribution of theconductive heat transfer conduits. The present invention thereforeprovides exchangers which are easier to build and which offer betterperformance in a variety of ways.

The design techniques described here can be applied to systems which useheat transfer fluids which are either liquid, gaseous or a mixture ofliquids and gases. It is particularly relevant however to systems usingliquid or gaseous heat transfer fluids.

The present invention therefore provides a method for designing heatexchangers with reduced hold up volumes of heat transfer fluid; the useof this smaller amount provides the following benefits:

-   -   a more reliable measure of heat balance is possible since the        temperature change of the heat transfer fluid is large and        therefore easier to measure. This provides valuable and accurate        information about the rate and progress of any operation which        liberates or absorbs heat. The effects of thermal change within        the heat transfer fluid are also reduced;    -   a small inventory of heat transfer fluid with a comparatively        short flow path can be replaced more quickly leading to improved        temperature control;    -   better transmission of heat from the heat transfer fluid to the        heat transfer surface is possible with the reduced volume        design;    -   better distribution of heat transfer fluid is possible due to        the use of multiple small pipes.

In a first embodiment the present invention therefore provides a heatexchanger for delivery of heat transfer fluid to a process heat transfersurface which is in contact with a process fluid wherein the heattransfer surface comprises at least five heat transfer conduits eachhaving a cross sectional area for the flow path of less than 2000 squaremillimetres wherein the linear velocity of the heat transfer fluidthrough the heat transfer conduits is from 0.5 to 20 m/s and adapted sothat the temperature of the heat transfer fluid changes by at least 1°C. when the system is operating at full load.

In a second embodiment the present invention therefore provides aprocess for the transfer of heat between a process fluid and a heattransfer fluid across a heat transfer surface in which the heat transfersurface comprises at least five heat transfer conduits each having across sectional area for the flow path of less than 80 squaremillimetres wherein the linear velocity of the heat transfer fluidthrough the heat transfer conduits is from 0.5 to 20 m/s and thetemperature of the heat transfer fluid changes by at least 1° C. whenthe system is operating at full load.

It is preferred that the heat transfer fluid have a relatively shortresidence time in the conduits. We prefer that this residence time inseconds is not greater than twice the length of the heat transfersurface as measured in metres.

The key to this design can be summarised in the following statements:

-   -   the heat transfer fluid is delivered to the heat transfer        surface in multiple small elements. Typically a single element        would not carry more than 20% (and in many cases much less) of        the total heat transfer fluid.    -   there is no need to maximise turbulence in the heat transfer        conduit in order to achieve good transmission of heat between        the heat transfer fluid and the process material. Most reduced        volume heat exchangers of the present invention operate with the        heat transfer fluid flowing under laminar conditions (although        it can be turbulent in some cases).    -   maximising turbulence in the heat transfer conduit is not a        design requirement for achieving good distribution of heat        transfer fluid.    -   the heat transfer fluid may be carried within a fully contained        conduit or pipe and, this pipe or conduit may be of an entirely        different material to the containment wall for the process        material particularly where the heat transfer element is an        external heat transfer element. This can avoid the time        consuming and costly welding required when half coils are used        as external heat transfer elements.    -   where external heat transfer elements are used, the heat        transfer fluid conduit wall may be used to enhance the        efficiency of heat transmission and maintain even distribution        of heating or cooling.    -   in the case of external heat transfer elements, the full        hydraulic perimeter of the containment conduit for the heat        transfer fluid is used for transferring heat to and from the        heat transfer fluid.    -   the shape of the containment conduit for the heat transfer fluid        can be modified. to increase the heat transfer area between the        conduit and the heat transfer fluid.    -   each heat transfer element may be sized such that, at maximum        heating or cooling load, the heat transfer fluid undergoes a        significant temperature change (typically greater than 1° C.)        whilst flowing at a comparatively high linear velocity        (typically 0.5 to 5 m·s⁻¹). In many cases this temperature        change will be >3° C.

In a further embodiment of the present invention which employs externalheat transfer elements which are of different material to the processcontainment vessel, the design can be developed to allow fordifferential expansion due to changes in temperature and/or pressure ofthe heat transfer surface and the conduit carrying the heat transferfluid. The invention therefore provides techniques whereby stressproblems created by differential expansion can be overcome by the use ofthermally conductive expansion plates as described herein below.

PCT/EP02/09956 and GB 2374 948 describe a design for variable area heatexchangers. The heat transfer surfaces described in those patents arebroken up into multiple separate elements. In sizing individual heattransfer elements, the linear velocity and the temperature change of theheat transfer fluid passing through the heat transfer element areimportant design criteria. By using a multi element variable area heatexchanger, these earlier patents are concerned with more effective useof heat transfer fluid. It was also shown that such heat exchangersoffer (amongst other things) more accurate heat measuring capabilitiesand better temperature control.

The present invention exploits the concept of multi-element heatexchangers to achieve reduced hold up volumes of heat transfer fluid. Aswith the variable area design, specified values of temperature changeand velocity of the heat transfer fluid are design requirements. Unlikethe earlier variable area design however, the present inventionregulates the log mean thermal difference (between the process materialand the heat transfer fluid) to control temperature. In the design ofthe present invention the full heat transfer area or large sections ofheat transfer area carry a continuous flow of heat transfer fluid. Inthe design of the present invention, the prime purpose of using multipleelements is to provide good heat transmission and even distribution ofheating or cooling fluid whilst maintaining small hold up volumes ofheat transfer fluid.

On very small heat exchangers, some of the design criteria of thepresent invention are less easy to differentiate from those of thetraditional systems. For example a very small heat exchanger willusually have laminar flow by default. However even on these very smallheat exchanger systems some of the key design criteria of the presentinvention enhance the performance of the heat exchanger. The mostimportant being the division of the delivery of the reduced volumes ofthe heat transfer surface into multiple elements.

On large heat exchangers, the differences between the present inventionand conventional designs are very clear cut. Although some reactors usemultiple heat transfer elements they are used for different reasons. Insome existing systems, separate heat transfer elements are used to allowdifferent sections of the equipment to be heated or cooled separately.In other cases, the heat transfer elements are broken up to reduce thetemperature change of the heat transfer fluid. In the reduced volumedesign, one of the design objectives is the very converse of this. Theheat transfer surface is broken up so as to give a comparatively largetemperature change in the temperature of the heat transfer fluid.

The amount of heat which can be transmitted by a heat exchanger isdetermined by the equation:q=U·A·LMTDWhere

-   -   q=heat transmitted (W)    -   U=heat transfer coefficient (W·m⁻²·K⁻¹)    -   A=the heat transfer area (m²)    -   LMTD=temperature difference between heat transfer fluid and        process material (K)

The components of this equation and their implication for heat exchangerdesign are discussed below.

The heat transfer coefficient (U) defines the ease with which heat canbe transmitted between the heat transfer fluid and the process material.

FIG. 5 shows a heat transfer surface between two fluids. For good heattransmission, the heat transfer surface (X_(b)) in FIG. 5) should be asthin as possible and have high thermal conductivity. In practice howeverwall thickness and choice of material are usually governed by the needto maintain adequate mechanical strength and resistance to chemicalattack.

Boundary layers lie at the interface between the heat transfer fluid andthe conduit and between the process material and the heat transfersurface. In these boundary layers, virtually no mixing occurs and heathas to cross by conduction. The thickness of the boundary layers(X_(HTF) and X_(p) of FIG. 5) reduces as the turbulence in therespective bulk fluids increase. Thin boundary layers have reducedresistance to heat transmission. Traditionally, large heat exchangerspromote high turbulence to reduce the thickness of the boundary layer.

The amount of heat that can be transferred is directly proportional tothe area of the heat transfer surface available (A). Folded surfacesoffer greater heat transfer area but the scope for this is limited formany applications. Ease of cleaning and the need to avoid stagnantpockets are key design considerations. In some cases however limitedprofiling is used (such as dimples or ribs) to increase heat transferarea (this can also help to promote turbulence).

The amount of heat that can be transferred is directly proportional tothe difference in temperature between the process material and the heattransfer fluid. The average of the temperature difference between therespective materials is referred to as the Log Mean Thermal Difference(LMTD). Variation of the LMTD is used as the temperature controlparameter in fixed area heat exchangers whereas variable area heatexchangers such as those described in PCT/EP02/09956 and GB 2374948, theLMTD can be kept substantially constant.

In conventional heat exchangers the size and shape of jacketed vesselsare largely determined by functional requirements such as productcapacity needs, the need for uniform agitation within the processmaterial, velocity control within the process material and ease ofcleaning. For these reasons many heat exchangers have relatively simpleinternal geometry. The heat transfer surface is often formed around theouter surface of the vessel. In some cases, one or more internal coilsor plates are fitted.

The designer seeks to maximise the heat transfer capacity by improvingthe heat transfer coefficient. In conventional large heat exchangers,this is achieved by maximising turbulence. A measure of how turbulent asystem is can be related to the Reynolds number as follows:Re=ρ·v·dl/μWhere

-   -   Re=Reynolds number [dimensionless]    -   ρ=Fluid density [kG·m⁻³]    -   v=fluid velocity [m·s⁻¹]    -   μ=fluid viscosity[N·s·m⁻²]

As a general rule, the flow conditions of fluids turn from laminar toturbulent at a Reynolds number of around 2000. Above 2000 turbulencetends to increase with rising Reynolds number and with this increase theboundary layer gets thinner thus reducing resistance to heattransmission. Conventional large heat exchangers maximise turbulence byincreasing the liquid velocity. In the case of jacketed systems,turbulence is promoted by injecting heat transfer fluid in to the jacketat high velocity or with the use of baffles. In the case of coils ortubes, heat transfer fluid is pumped at high velocity.

The present invention on the other hand uses reduced volumes of heattransfer fluid in a manner that secures a reasonable temperature drop ofthe heat transfer fluid whilst keeping the fluid velocity at anacceptable high level. The heat transfer fluid flow on large systems(with a nominal heat transfer capacity of greater than 3 kW per heattransfer conduit based on a temperature rise of 5° C. in the heattransfer fluid) usually has reduced turbulence with a Reynolds number ofless than 10,000. On mid sized systems (with a nominal heat transfercapacity of greater than 0.1 to 3 kW per heat transfer conduit based ona temperature rise of 5° C. in the heat transfer fluid) the Reynoldsnumber is usually below 2000. On small systems (with a nominal heattransfer capacity of less than 0.1 kW per heat transfer conduit based ona temperature rise of 5° C. in the heat transfer fluid) the Reynoldsnumber is usually less than 500. It must be recognized however, thatsystems can deviate from the typical values shown above especially wherevery high pressure drops through the heat transfer conduit are used. Itis generally preferable however that the pressure drop of the heattransfer fluid is not excessive (>2 bar). There are however differentdesign considerations depending on whether the heat transfer elementspass through the process material or are wrapped around the externalsurface. The two cases are considered separately below.

Reduced Volume Systems with External Heat Transfer Elements

The amount of heat removed by the heat transfer fluid can be calculatedfrom the following equation.q=mCp(t _(si) −t _(so))Where

-   -   q=heat transfer capacity of heat exchanger (W)    -   m=mass flow of heat transfer fluid (kg·s⁻¹)    -   Cp=specific heat of heat transfer fluid (J·kg⁻¹·K⁻¹)    -   t_(si)−t_(so)=temperature change of heat transfer fluid (K)

Where t_(si) is the temperature of the heat transfer fluid on entry untothe heat exchanger and t_(so) is the temperature of the fluid on existfrom the heat exchanger.

In the reduced volume system of the present invention, the linearvelocity and the temperature change of the heat transfer fluid are keydesign parameters. The velocity and temperature change of the heattransfer fluid (t_(si)−t_(so)) are preferably as large as is reasonablypractical. In addition to this, it is preferable to use the lowestacceptable design heat load. Some of the design considerations whicharise from these objectives are considered below.

Consideration 1: The Design Heat Load (q_(des))

The design heat load will be based on the nominal maximum heat load ofthe process whose temperature is to be controlled. If the equipment isused for multiple purposes, this will be based on the process operationwhich has the highest heat load. For example biological processes tendto have a lower maximum heat load than chemical processes. In some casesthe design heat load may be based on emergency conditions. It should benoted that some systems can be designed on the basis that short termpeak heat loads may be higher than the maximum steady state coolingcapacity of the system. In this type of case, a design heat capacitywhich is smaller than the peak process heat capacity can be beneficialsince it will require a smaller inventory of heat transfer fluid. Thedesign heat load is referred to as q_(des).

It is good practice to choose a q_(des) which is larger than theabsolute value of the maximum heat load. This additional safety marginallows for errors in calculation or unforeseen operating conditions.

Consideration 2: The Heat Transfer Fluid Temperature Rise(t_(si)−t_(so)),

The maximum acceptable value of heat transfer fluid temperature rise(t_(si)−t_(so)), should be established bearing in mind the operatingconditions and capabilities of the system.

The heat transfer coefficient (U) needs to be established. The U valuedefines how easily heat can be transmitted between the process materialand the heat transfer fluid. This can be calculated using standard heattransfer theory but is most easily taken from test data or historicaldata.

The nominal area of the heat transfer surface (A) of the heat exchangerneeds to be established. For the purposes of this description, this canbe based on the heat transfer area in contact with the process material.It should be noted that this is not the true heat transfer area sincethe heat transfer area in contact with the heat transfer fluid is likelyto be different to the heat transfer area in contact with the processmaterial.

Thus using q_(des), U and A, the design value for the log mean thermaldifference can be determined as follows:q _(des) =U·A·LMTD _(des)Where

-   -   q_(des)=the maximum process heat load (W)    -   U=the heat transfer coefficient (W·m⁻²·K⁻¹)    -   A=the nominal heat transfer area (m²)    -   LMTD_(des)=Design log mean thermal difference (K)

The LMTD_(des) can now be used to establish a design value for the heattransfer fluid temperature change (t_(si)−t_(so)). The LMTD is the trueaverage difference between the heat transfer fluid temperature and theprocess temperature and is calculated from the following formula:LMTD=[(T _(p) −t _(si))−(T _(p) −t _(so))]/ln[(T _(p) −t _(si))/(T _(p)−t _(so))]Where

-   -   T_(p)=process temperature and is generally fixed    -   t_(si)=inlet temperature of the heat transfer fluid    -   t_(so)=exit temperature of the heat transfer fluid as        illustrated in FIG. 6.

By testing different values of T_(p)−t_(si), alternative values oft_(si)−t_(so) can be found which when calculated out, give the designLMTD (LMTD_(des)). In this evaluation the following factors need to beconsidered:

-   -   the value of t_(si) must not be so high that it causes heat        damage or unwanted boiling of the process material. The        temperature (t_(si)) must also fall within the design        capabilities of the system.    -   the value of t_(si) must not be so low that it causes cold        damage or unwanted freezing of the process material. The        temperature (t_(si)) must also fall within the design        capabilities of the system.    -   a high value of t_(si)−t_(so) reduces the maximum heat transfer        capacity but improves the ability to measure heat balance        accurately.    -   a low value of t_(si)−t_(so) requires more pumping energy to get        the heat transfer fluid through the heat exchanger.

We prefer that t_(si)−t_(so) is in the range of 0.1° C. to 1000° C. Amore normal design range of t_(si)−t_(so) however would be between 1° C.and 20° C., preferably between 3° C. and 20° C. more preferably between3° C. and 15° C.

The design value of t_(si)−t_(so) could be determined by other criteria.For example, the designer may need to measure the heat balance of theworking system. If he has a temperature measuring device which iscapable of measuring to ±0.1° C., he might select a t_(si)−t_(so) of 5°C. on the basis that this gives a temperature measuring accuracy of ±2%.

As a further embodiment therefore the present invention provides the useof a predetermined t_(si)−t_(so) of a heat transfer process fluid to beemployed in a heat exchanger for the design of the heat exchanger so asto reduce the hold up volume of heat transfer fluid within the heatexchanger.

In a preferred system the hold up volume is reduced to the minimumacceptable volume as determined by the nature of the heat exchanger.

Consideration 3: The Heat Transfer Fluid Flow Rate (m)

Having determined the design heat load (q_(des)) and the designtemperature change of the heat transfer fluid (t_(si)−t_(so)) therequired rate of flow of heat transfer fluid at maximum load can becalculated from the following equation.m=q _(des) /[Cp(t _(si) −t _(so))]Where

-   -   q_(des)=design heat load    -   m=mass flow of heat transfer fluid (kg·s⁻¹)    -   Cp=specific heat of heat transfer fluid (J·kg⁻¹·K⁻¹)    -   t_(si)−t_(so)=temperature change of heat transfer fluid (K)

The mass flow of heat transfer fluid (m) may be used as one of thefactors for sizing the heat transfer elements. In practice however thisis not a fixed value since it can be varied by increasing or decreasingthe velocity of heat transfer fluid through the heat transfer elements.The ability to vary the velocity (and temperature) of the heat transferfluid is useful and gives the equipment operator the freedom to vary theheat exchanger performance around a core design value.

As a further embodiment the present invention therefore provides the useof a predetermined linear velocity of the heat transfer fluid to beemployed in a heat exchanger for the design of the heat exchanger so asto reduce the hold up volume of heat transfer fluid within the heatexchanger to the minimum acceptance volume.

In a preferred system the hold up volume is reduced to the minimumacceptable volume as determined by the nature of the heat exchanger.

Consideration 4: The Number of Heat Transfer Elements (n)

The underlying purpose of this invention is to minimise the volume ofheat transfer fluid held within the heat exchanger. It is also desirableto utilise the maximum available heat transfer area. The thickness ofthe heat transfer fluid layer surrounding the heat exchanger can becalculated as follows:

At any time, the volume of heat transfer fluid in service is as follows:V=m·r/ρWhere

-   -   V=total volume of heat transfer fluid (V)    -   m=mass flow of heat transfer fluid (kg·s⁻¹)    -   ρ=density of the heat transfer fluid (kg·m⁻³)    -   r=residence time of the heat transfer fluid (s)

The residence time (r) is calculated from the following relationship:r=Z/vWhere

-   -   r=residence time of heat transfer fluid (s)    -   Z=total length of each heat transfer fluid conduit (m)    -   v=Velocity of heat transfer fluid (m·s⁻¹)

Although there are aspects of Z and v which may have to be tested byiterative methods, simple criteria may often be applied. For example, ona cylindrical vessel Z will often be a simple multiple of the vesselperimeter (half, once or twice the distance of the perimeter) to makefabrication simple. The velocity may also be set in say the range of 1to 3 m·s⁻¹, to deliver fast temperature control response without beingso high as to incur excessive pressure drop.

Thus the thickness of the heat transfer fluid layer can be calculated asfollows:W=V/AWhere

-   -   W=thickness of the heat transfer fluid layer (m)    -   V=total volume of heat transfer fluid (m³)    -   A=heat transfer area (m²)

The heat transfer area referred to is that which is in contact with theprocess material. The reduced volume concept seeks to reduce the heattransfer fluid to a thinnest possible layer (W).

One consequence of designing a minimum volume system is that thethickness of the heat transfer fluid layer surrounding the heat transfersurface is reduced. If a single thin sheet of fluid is used however, thefluid will tend to channel and not give uniform distribution. As thislayer gets thinner, problems of fluid distribution arise and the heattransfer fluid starts to channel as shown in the transition from FIG.7(a) to FIG. 7(b).

The solution to the channelling problem is resolved by breaking up theheat transfer surface into separate channels as shown in FIG. (8). It ispreferable that the ratio of height to width of these channels islimited such that one dimension is not more than five times the other,i.e. L is no more than five times W in the system illustrated in FIG. 8.

In the full pipe version of the minimum flow design, the side walls ofthe conduits not in contact with the heat transfer surface also serve asconductors thus giving a second reason for a low L:W ratios. In systemswhere there are very small channels (<1 mm² in cross sectional area) ormultiple small channels, the internal dimensions may use ratios of up to10:1 (L to W or W to L) where multiple conduits are arranged in parallelas illustrated in FIG. 9, L can be 10 times W or less. In this case, Wis calculated as follows:W=W1+W2+W3 . . .depending on how many elements are used.

This relationship applies irrespective of the shape of the conduit andthree different shapes of conduit are illustrated in FIG. 10.

There will be different ways of determining the number of heat transferconduits according to the geometry and layout of the heat transfersurface. In the case of a cylindrical vessel (with heat transferconduits only on the cylindrical side), with conduits around thecylinder of one full turn, the theoretical minimum number of conduitswould be:n _(min) =H/(L−Y)Where

-   -   H=height of vessel cylinder (m)    -   W=thickness of the heat transfer fluid layer (m)    -   Y=wall thickness of the heat transfer fluid conduit (m)

This design process is indicative and not absolute and there is noabsolute rule for the ideal number of elements since the fabricationmethod chosen will have a significant impact on how the elements areassembled and how they perform. There are, however, some generalconsiderations to take into account:

-   -   larger systems require a greater number of elements.    -   the elements need to cover adequate heat transfer surface area        in contact with the process material.    -   as a general rule, a larger number of elements will permit        better distribution of heat transfer fluid and better        transmission of heat.    -   if the number of elements is too large, their individual flow        capacities will reduce to a point where blockage can be a        problem.

The number of elements used on a system can vary from 5 to manythousands. A typical number however will vary from 5 to 200 depending onthe size of the system, prefered systems contain for 10 to 200, mostpreferably 10 to 100, most preferably 10 to 50 elements.

The design process may be iterative and the number of elements chosenmay need to be reconsidered several times during the design process.

Consideration 5: The Heat Transfer Element Cross-Section (d)

High linear velocities (of heat transfer fluid) are desirable as theygive small hold up volumes of heat transfer fluid. They also improve theresponse time for temperature control.

Having specified a design value for the linear velocity, the crosssectional area of flow can be calculated from the following equationa _(e) =m/[v·ρ·n]Where a_(e)=cross sectional area of flow path (m²)

-   -   m=mass flow of heat transfer fluid (kg·s⁻¹)    -   v=linear velocity of heat transfer fluid through the conduit        (m·s⁻¹)    -   ρ=density of heat transfer fluid (kg·m⁻³)    -   n=number of elements used

If the heat transfer element were circular, this would give an internalpipe diameter of:d=[4a _(e) /II)^(1/2)Where

-   -   d=Internal diameter of heat transfer element (m)    -   a_(e)=cross sectional area of flow path (m²)

In practice, the conduits carrying the heat transfer fluid may have avariety of different cross sectional shapes.

The cross sectional area is dependent on the size of the heat transfersurface and the heat load it is required to carry it should however beless than 2000, preferably less than 1300, more preferably less than500, more preferably less than 80 and in some instances less than 20perhaps less than 1 square millimetre.

Linear velocities of between 0.01 m·s⁻¹ and 10 m·s⁻¹ can be used fordesign purposes in liquid cooled systems. In practice however very lowvelocities are undesirable as they yield large hold up volumes in theheat transfer conduit. The use of large volumes also increases theresponse time for temperature control which is undesirable. Very highvelocities can also present problems as they create high pressure dropsthrough the heat transfer element which can put too high a load on thepumps that deliver the fluid. A typical system will be designed forlinear velocities of between 0.5 m·s⁻¹ and 5 m·s⁻¹. In the case ofsystems heated or cooled by gas, the maximum range can vary from 0.1m·s⁻¹ and 100 m·s⁻¹ but a typical range will be 2 m·s⁻¹ to 20 m·s⁻¹. Itis preferred that the pressure drop across the heat transfer element beno greater than 10 bar and is preferably in the range 0.5 to 5 bar.

Consideration 6: The Heat Transfer Element Length (Z)

Once the cross sectional areas for the heat transfer elements have beenestablished, the optimum lengths can be determined. Each heat transferelement has an optimum heat carrying capacity. If the element is tooshort, the heating/cooling capacity of the heat transfer fluid will beunder utilised. If the heat transfer element is too long, the heatcarrying capacity of the element will be insufficient for the heattransfer area covered.

The nominal heat transfer capacity of each heat transfer element may becalculated as follows:q _(e) =q _(des) /nWhere

-   -   q_(e)=nominal heat transfer capacity of each element (V)    -   q_(des)=design heat load for the system (W)    -   n=number of heat transfer elements used.

Each heat transfer element has to cover a specific area of heat transfersurface. This area is calculated as follows:A _(e) =q _(e) /U·LMTD _(des)Where

-   -   A_(e)=area covered by the heat transfer element (m²)    -   q_(e)=nominal heat transfer capacity of each element (W)    -   U=the heat transfer coefficient (W·⁻²·K⁻¹)    -   LMTD_(des)=design log mean thermal difference (K)

As an approximation, the nominal width (w) of the heat transfer elementcan be taken as the distance between the centre line of one heattransfer element and the centre line of the next heat transfer element.From this, the nominal length of the element can be calculated asfollows:z _(e) =A _(e) /w _(e)Where

-   -   zl_(e)=length of each heat transfer element (m)    -   A_(e)=nominal heat transfer area of each element (m²)    -   w_(e)=nominal width of each heat transfer element (m)

Once the length of the heat transfer element has been established, thepressure drop should be checked by conventional calculation methods. Ifthe pressure drop is too high, the calculation may need to be repeatedwith a larger cross sectional flow areas and/or different flow pathlengths. This may in turn lead to choosing a different number of heattransfer elements. The evaluation process may require a number ofiterations. In many cases it will be preferable to start with anominated length of heat transfer element as one of the design criteria.

For external heat transfer conduits, the maximum effective path lengthof a heat transfer conduit is preferably less than twice the length ofthe heat transfer surface and more preferably is approximately equal tothe heat transfer surface of the heat exchanger.

In the case of a cylindrical vessel, the length of the heat transfersurface is:P=π·DWhere

-   -   D=diameter of the cylinder    -   p=3.1416    -   P=perimeter of heat exchanger

This gives a maximum residence time of:r=D/vWhere

-   -   r=residence time (s)    -   D=diameter of the cylindrical vessel (m)    -   V=linear velocity of the heat transfer fluid (m·s⁻¹)

On the basis that the preferred minimum velocity is 0.5 m·s⁻¹, thislimits the residence time of heat transfer fluids in reduced volumesystems. Thus, the maximum residence time for a 3 metre diametercylindrical vessel with a single loop would be approximately 19 secondswith a fluid velocity of 0.5 m·s⁻¹. With a preferred linear velocity ofsay 2 m·s⁻¹, the residence time would be under 5 seconds. Typically theresidence time of heat transfer fluid in a reduced volume heat exchangerwill be from 0.1 to 5 seconds according to the size of the system. Verysmall (under 1 litre) and very large systems (such as cylindricalvessels of greater than 5 metre in diameter) may have residence timesabove and below the typical value described above. The heat transferconduits area generally arranged so as to lie parallel with thehydraulic plane selected. In other shapes of vessel, a different planemay be chosen to define the length of the heat transfer surface. Theplane which is used to define the length of the heat transfer surfacehowever must have sufficient length to ensure efficient heat transferwithout an unmanageably large number of heat transfer conduits. In somecases such as narrow pipes the conduit may be wrapped in such a was asto be more than twice the length of the heat transfer surface.

The length of the heat transfer surface varies according to the geometryof the vessel with which it is used. In the case of a cylindrical vesselthe length may be the perimeter or the height of the cylinder, in thecase of a conical vessel the length may be the perimeter of an upperportion of the cone. In the case of a square of rectangular vessel thelength is the surface with which the conduit is in contact.

The residence time of the heat transfer fluid depends upon the size ofthe system but it should be from 0.01 to 100 seconds. In small systemswhere the transfer surface is not greater than 10 metres in length,preferably the residence time of heat transfer fluid is less than 6seconds, preferably less than 5 seconds and more preferably less than 4seconds, most preferably less than 3 seconds and is in the range 0.01 to6 seconds.

Under these conditions different cross sectional areas and internalprofiles of the heat transfer element need to be tested to find theoptimum hydraulic design. Developing the design on the basis of definedlengths of heat transfer element can simplify the mechanical. If forexample, each element covers half, one or two full loop of a cylinder;the manifolding will be simpler and neater.

Consideration 7: Other Calculation Considerations

The methodology described above has been simplified for the purpose ofsetting out the key design objectives. In practice more rigorous methodscan be used to good effect. For example, in the method shown, the Uvalue was assumed and the area was based on the surface area in contactwith the process material. An alternative analysis would employ anincremental approach using the following equation:1/UA=1(h _(htf) ·A _(htf))+L _(c)/(k _(c) ·A _(c))+L _(p)(k _(p) ·A_(p))+1/(h _(p) ·A _(p))Where

-   -   UA=nominal heat transfer capacity per Kelvin (W·K⁻¹)    -   h_(htf)=heat transfer fluid coefficient (W·m⁻²·K⁻¹)    -   A_(htf)=heat transfer area of the internal conduit wall (m²)    -   L_(c)=thickness of the conduit wall (m)    -   K_(c)=thermal conductivity of the conduit wall (W·m⁻¹·K⁻¹)    -   A_(c)=Contact area between the conduit and the process wall (m²)    -   L_(p)=thickness of the process wall (m)    -   K_(p)=thermal conductivity of the process wall (W·m⁻¹·K⁻¹)    -   A_(p)=Area of the process wall (m²)    -   H_(p)=Process material heat coefficient (W·m⁻²·K⁻¹)

In the previous analysis it was also assumed that the wall thicknesseswere consistent. In practice the thickness of the conduit (L_(c)) wallmay be varied or extended to ensure that there is good continuity fortransmission of heat between the heat transfer fluid and the processmaterial. An illustration of this is shown in FIGS. 11 and 12 where goodcontinuity is provided between the heat transfer fluid and the processmaterial. FIG. 20 shows how this can be done even when expansion plates,as described hereafter, are employed.

Design of reduced volume systems can be an iterative process and somecompromise may have to be made (e.g. t_(si)−t_(so) and fluid velocitymay not be ideal). The method described above is not therefore intendedto represent a definitive or rigorous design approach but providessufficient information to enable a design to be established. Thedesigner may use a variety of techniques for reaching a solution;however the underlying objective is to deliver a heat exchanger whichrelies on much smaller inventories of heat transfer fluid than istypical for conventional heat exchangers.

It should be recognised that some systems will not require heatmeasurement and comparatively high values of t_(si)−t_(so) can betolerated. Even in these cases however, a value which is slightly highervalue than might be used in a comparable conventional heat exchanger isbeneficial since it serves to reduce the hold up volume within the heatexchanger with negligible loss of heat transfer capacity. By applyingthe reduced volume design principles, fast temperature response isachieved and a simpler fabrication method is possible.

For a fixed area heat exchanger, the ideal performance will only holdgood for one set of operating conditions. Even with this compromisehowever, the design of the present invention will deliver a system whichis simple to build and generally closer to an optimum design than mostconventional heat exchangers.

Consideration 8: Conduit Design

In most conventional heat exchangers, excess heat transfer fluid is usedto achieve uniform fluid displacement and turbulence throughout the heattransfer conduit.

In the reduced volume design of the present invention, flow conditionsare often laminar, particularly with small systems, and severaldifferent techniques are used to enhance the heat transfer coefficientand ensure even distribution of heating or cooling fluid.

It is desirable to use the maximum available area of heat transfersurface in contact with the process material. The heat transfer conduitsneed to be laid out in manner which ensures that the available area ofheat transfer surface is properly covered. For this reason, shapes andmaterials of conduits which can be adapted to the profile of the processheat transfer wall are preferably used.

In the case of external jackets or coils, the conduits used to carry theheat transfer fluid do not need to be compatible with the processmaterial and are not subjected to the same thermal or mechanicalconditions as the process. In these circumstances, a material of highthermal conductivity like copper makes an ideal conduit material and hasa thermal conductivity which can be more than 20 times greater than thatof stainless steel. Because the conduits are comparatively small, theentire internal wetted surface of the heat transfer fluid conduit can beused to transmit heat between the heat transfer fluid and the processheat transfer wall. Because the conduits carrying the heat transferfluid have to conduct heat across their walls, they are referred to asconductor pipes. Whilst it is desirable to fabricate conductor pipes inconductive material, less conductive materials can be made to work wellby virtue of their small size and adaptable shape. FIG. 11 showsexternal conductor pipes. In some cases (such as for accuratecalorimetry in variable area systems), it may be preferable to have anair gap or insulation between the individual heat transfer elements.

For external conductor pipes, a further increase in the heat transferarea (for the heat transfer fluid) can be achieved by using conductorpipes with an oblong profile as shown in FIG. 12.

FIGS. 11 and 12 show single conductor pipes with circular or rectangularcross sections. In practice however a variety of shapes and groups ofconductor pipes can be used to optimise the contact area between theheat transfer fluid and the conductor pipe whilst maintaining goodmechanical strength. Some examples are shown in FIG. 13.

The internal profile of the conductor pipe can also be modified in otherways to give an enhanced heat transfer area as shown in FIG. 14.

Additionally the heat transfer performance can also be improved by theaddition of dimples, knurling or other surface enhancement to the innerconduit wall.

Inserts can be used to good effect in reduced volume systems (as shownin FIG. 15). Smooth inserts can be used to reduce the internal hydraulicvolume of the conductor pipe. This allows a larger conduit to hold areduced volume of heat transfer fluid. By using different diameters ofinserts, common pipe sizes can be used for multiple flow duties. Largerconduits with removable inserts can also be used to make cleaningeasier. Profiled inserts (such as flow disrupters) can also be used topromote mixing and improve heat transfer conditions at the boundarylayer.

Consideration 9 Assembly of the Heat Transfer Conduits

External heat transfer elements can be fixed to the process heattransfer surface by a variety of methods. Reduced volume jackets can befabricated in the same manner as traditional half coil designs as shownin FIG. 16.

Whilst conventional fabrication techniques can be used, they can belabour intensive for fabrication and may not deliver the optimum thermalperformance. A preferred solution is to use fully contained conductorpipes as shown below. These pipes can be fixed to the heat transfersurface using adhesive, solder, brazing or welding. It is preferablethat the bonding agent has good thermal conductive properties. Thecontact area between the heat transfer conduit and the heat transfersurface can be as small or large as the designer deems necessary. Theexample in FIG. 17 uses bonding material on one side of the conductorpipe.

Alternatively conductor pipes can also be sprung or clamped onto theheat transfer surface as shown in the FIG. 18 (clamping arrangement notshown). The heat transfer capacity can be enhanced by filling the airspace between the conductor pipe and the heat transfer surface with asoft conductive layer such as conductive paste, fluid, conductive wool,or conductive matting. Composite layers could also be used such ascopper wool impregnated with conductive grease.

Conductor pipes can be fabricated in several sections as shown in theillustration below. The conductor pipes can be held in place by springsor clamps (not shown). As with the system illustrated in FIG. 18 a softconductive layer can be used to fill the air gap, FIG. 19 shows a twopart mounted conductor pipe configuration.

Another aspect of the present invention provides a method to reduce oravoid problems of differential expansion and/or contraction due tochanges in temperature and/or pressure which can cause stress if theconductor pipe is of a different material to the heat transfer surface.According to the invention this problem can be overcome by usingexpansion plates to link the conductor pipe to the heat transfersurface. The use of an expansion plate allows the conductor pipe toexpand at a different rate and/or to a different extent to the heattransfer surface. It is preferable (but not always essential) thatexpansion plates are fabricated in materials that have good thermalconductivity properties. An example of an expansion plate is shown inFIG. 20.

The present invention therefore further provides a heat transfer systemfor the transfer of heat between a process fluid and a heat transferfluid across a heat transfer surface comprising a heat transfer conduitfor passage of the heat transfer fluid attached to an expansion platesaid expansion plate being in contact with the heat transfer surfacesaid expansion plate enabling independent movement of the heat transferconduit and the heat transfer surface.

With the system shown in FIG. 20 the conductor pipe is free to move upand down in relation to the heat transfer surface.

The expansion plates for the conductor pipes can be made into a varietyof shapes to accommodate compact construction methods. For example theycan be wedge shape, notched or chamfered as shown in FIG. 21. An exampleof a slightly different expansion plate is shown in FIG. 22.

The expansion plate can be made up in several sections with theconductor pipe clamped, welded, braised, bonded or soldered to it.

In a further embodiment of the present invention the heat transferelement is mounted on an expansion plate and the expansion plate itselfis provided with a channel. The channel in the expansion plate beingadapted to receive a band which can hold the expansion plate against thesurface of the process vessel. In this way the expansion plate may beplaced against the surface of the process vessel, the band inserted andtensioned so as to force the expansion plate against the surface of thevessel and hold the heat transfer element in place against the surfaceof the vessel. In this way, good transmission of heat between the heattransfer fluid and the process material may be accomplished whilst theexpansion plate is free to alter its configuration independently of theprocess vessel as the temperatures and/or pressures increase or decreaseand there is differential expansion due to the different coefficients ofexpansion of the process vessel and the heat transfer element. Such asystem is illustrated in FIGS. 23 to 25 in which FIG. 23 shows a heattransfer element material on an expansion plate provided with a slot.FIG. 24 shows the heat transfer element of FIG. 23 mounted on theprocess vessel with the metal band fixing strip in place to hold theexpansion element against the surface of the process vessel. FIG. 25shows how the heat transfer element of FIG. 23 can move away from itsposition in FIG. 24 due to the independent expansion of the expansionplate.

The use of such a system also makes reactor construction and assemblymuch easier in that welding, soldering or the use of adhesives is notrequired to fix the heat transfer elements to the process vessel. Inaddition the heat transfer elements can be easily replaced if necessaryperhaps because of damage or the need to change the size or nature ofthe heat transfer element.

Consideration 10 Maintaining Uniform Flow in the Heat Transfer Conduits

In the system of the present invention even distribution of the heattransfer fluid to the multiple heat transfer elements is generallydesirable. For example, if a system has 10 heat transfer elements ofsimilar size and length, it would be desirable for each element toreceive about 10% of the flow of heat transfer fluid. The flowcharacteristics of similar elements can be different however due tosmall variations arising during fabrication.

However, it is preferred to prevent major flow variations. This can beaccomplished by providing a regulating valve to each heat transferelement. In this way the flow characteristics can be adjusted for eachconduit by adjusting the valve setting. Alternatively a restrictionorifice can be fitted to each conduit such that the restriction throughthe orifice is large in comparison to the flow resistance of the conduitFlow restrictors with similar pressure drop characteristics are simpleto fabricate and are easy to change. This makes the pressure dropvariations between the different conduits small in relation to theorifice resistance, thus giving substantially constant pressure dropcharacteristics on all elements of similar size and type.

Some systems may have different lengths and sizes of heat transferelement and in this instance the same flow balancing techniques can beapplied although the valves or restrictors may be set differently fordifferent heat transfer elements.

Reduced Volume Designs with Internal Heat Transfer Elements

The description above illustrates design and fabrication methods wherethe heat transfer elements are fixed to the external surface of theequipment. The present invention may also be applied to systems wherethe heat transfer fluid conduit passes directly into the process fluid(such as internal coils or plates).

For internal systems, a reduced hold up of heat transfer fluid is alsodesirable and this is achieved by using multiple internal heat transferelements rather than one or a few large coils.

For internal heat transfer elements, the same design criteria apply asdescribed above in relation to external heat transfer elements. Theanalysis is made simpler however by the fact that the conductor pipe isin direct contact with the process material. Fabrication is alsodifferent as the conductor pipes are free to expand and contract and donot have to be fixed in place other than for mechanical support;expansion plates may not therefore be required.

The following benefits are realised through use of the systems of thepresent invention employing external and/or internal heat transferelements.

-   1. Heat balance may easily be measured because with a process held    at constant temperature, the heat gained or lost by the process    material is the same as the heat gained or lost by the heat transfer    fluid. When using the techniques of the present invention the heat    gain or loss of the heat transfer fluid can be determined by    measuring the inlet and outlet temperature and the mass flow of the    heat transfer fluid. Thus:    q=m·Cp(t _(si) −t _(so))    -   Where        -   q=heat liberated or absorbed by the process (W)        -   m=mass flow of heat transfer fluid (kg·s⁻¹)        -   Cp=specific heat of heat transfer fluid (J·kg⁻¹·K⁻¹)        -   t_(si)−t_(so)=temperature change of heat transfer fluid (K)    -   The use of reduced volume heat exchangers of the present        invention give better heat balance data by virtue of having        generally larger temperature changes in the heat transfer fluid        (which makes measuring the temperature change easier) and having        lower thermal inertia within the heat transfer fluid (changes in        thermal conditions of the jacket tend to mask the true heat        change within the process).    -   Even on systems where the process temperature can vary or there        is a phase change (e.g. crystallisation or boiling) in the        process material, useful heat balance data can still be        extracted.    -   Heat balance measurement is a valuable monitoring tool for many        chemical and physical processes. It can for example reveal the        rate and progress of chemical reactions. This, in turn, permits        the user to optimise addition rates and reaction times. In some        cases it serves a valuable safety function as heat monitoring        can detect the onset of runaways or accumulation of un-reacted        material. Heat monitoring can also be used to monitor and        control a variety of other processes such as crystallisation,        cell growth systems and drying.-   2. Temperature control can be improved. Good temperature control    requires fast response and a key factor in fast response is the    speed with which the heat transfer fluid temperature within the heat    exchanger can be modified. The heat transfer elements in a reduced    volume heat exchanger have comparatively short flow paths and the    fluid travels as a plug through the heat transfer element. High    linear velocities of the heat transfer fluid are also used. These    conditions all serve to give fast temperature control response.-   3. On the external conductor pipe design, the heat transfer capacity    can be increased since the are of the heat transfer surface between    the heat transfer fluid and the conductor pipe can be increased to a    value which is greater than the process surface area in contact with    the process fluid. Because the heat transfer elements are small and    adaptable, they can be used to cover areas that a normal Jacket or    half coil could not reach (such as peripheral components attached to    the vessel). The higher heat transfer capacity and better surface    coverage will be of benefit where heat transfer capacity is    important.    -   Heating and cooling capacity constraints are a common problem in        many applications. In batch reactions for example, the addition        rate of reactant usually has to be reduced to the point where        the heat of reaction does not exceed the cooling capacity of the        system. In large fermenters, external jackets are often        inadequate for cooling purposes and internal coils have to be        added.-   4. The reduced volume design as described herein offers a very    simple construction method. Reduced volume heat exchangers will be    simpler and cheaper to fabricate than most comparable conventional    heat transfer devices, especially those with external jackets or    coils. Individual heat transfer elements are also simple to repair    or replace.-   5. The present invention allows for energy savings for the same    degree of heat transfer through reduced volumetric flow and reduced    pressure drop of heat transfer fluid through the heat transfer    conduit.-   5. The expansion plate concept used with external heat transfer    elements according to the present invention provides the additional    benefit that the heat transfer elements may be made of a different    material from the process vessel without causing problems due to    differential expansion of the materials. This then allows materials    with high thermal conductivity, such as copper, to be used for the    heat transfer elements.-   6. The reduced volume concept requires less volumetric flow through    the jacket than conventional heat exchangers and in many cases will    also be used with reduced pressure drops (of the heat transfer fluid    through the conduit). This will have the benefit of reducing the    size of pipework around the heat exchanger and will contribute    towards reduced energy requirements.    -   The reduced volume design and fabrication technique of the        present invention can be used for any heat transfer application        where better temperature control, better distribution of heat        transfer fluid, better heat transfer coefficients or better        measurement of heat balance is required. It can also be used to        simplify construction of heat exchangers where external jackets        or coils are used. It can also be used to save energy. This        technique will deliver a better design for industrial process        equipment such as reactors, dryers, mixers, fermenters, mills,        cell growth vessels, filters or extruders. It can also used for        direct fired equipment.    -   The technique can be employed on any size of system where the        inventory of process material is from 1 millilitre to 100,000        litres or even larger.    -   This design concept can also be applied to a variety of other        types of equipment which are not used within the process        industries. Examples included refrigeration systems, combustion        engines, hydraulic systems, heat exchangers in nuclear reactors,        air craft heating and cooling systems, heating and cooling        systems for ships, heating and cooling systems for road        vehicles, HVAC systems etc.

The invention can be used to improve the operation of laboratory scaleand commercial chemical and physical reaction systems. It can howeveralso be used to provide considerably smaller reaction systems withcomparable commercial throughput where calorimetric data permits theprocess to operate in continuous or semi-continuous manner. For examplethe invention enables reduction of reactor size by a factor of 10 and,in some instances, a factor of 100 or greater.

The invention is particular useful in the follow reactions

-   -   batch organic synthesis reactions currently carried out in        reactors of 10 to 20,000 litres.    -   bulk pharmaceutical synthesis reactions currently carried out        with process material quantities of 10 to 20,000 litres.    -   batch polymerisation reactions currently carried out in reactors        of 10 to 20,000 litres.    -   batch synthesis reactions of 10 to 20,000 litres currently used        for unstable materials (compounds susceptible to        self-accelerating runaways)    -   batch inorganic synthesis reactions currently carried out in        reactions of 10 to 20,000 litres.    -   evaporates, batch dryers, holding tanks, crystallisers,        fermenters, cell growth vessels, mills, mixers and filters        typically carried out in systems of 10 to 20,000 litres    -   compressors, internal combustion engines, air conditioning        systems

The techniques may also be useful in larger scale chemical andpetrochemical operations.

1. A heat exchanger for delivery of heat transfer fluid to a processheat transfer surface which is in contact with a process fluid whereinthe heat transfer surface fluid is delivered in at least five heattransfer conduits each having a cross sectional area for the flow pathof less than 2000 square millimetres wherein the linear velocity of theheat transfer fluid through the heat transfer conduits is from 0.5 to 20m·s⁻¹ and adapted so that the temperature of the heat transfer fluidchanges by at least 1° C. when they system is operating at design heatload.
 2. A heat exchange according to claim 1 in which the time takenfor the heat transfer fluid to pass through the heat exchanger asmeasured in seconds is not greater than twice length of the heattransfer surface when said length is measured in metres.
 3. A heatexchanger according to claim 1 in which the conduits have a crosssectional area for the flow path of less than 180 square millimetres. 4.(canceled)
 5. A heat exchanger according to claim 1 where the heattransfer fluid is delivered in 5 or more separate heat transfer fluidconduits where the total inventory of gas, liquid or solid to be heatedor cooled within the device is less than 1000 litres.
 6. A heatexchanger according to claim 1 where the heat transfer fluid isdelivered in 3 or more separate heat transfer fluid conduits per 1000litres of gas, liquid or solid to be heated where the total inventory ofsaid gas, liquid or solid within the heat transfer device is greaterthan 1000 litres.
 7. (canceled)
 8. (canceled)
 9. A heat exchangeraccording to claim 1 wherein the linear velocity of the heat transferfluid through the heat transfer conduit is between 0.5 and 5 m·s⁻¹ forliquid cooled systems when the heat exchanger is operating at fulldesign load and between 2 and 20 m·s⁻¹ for gas cooled systems when theheat exchanger is operating at full design load.
 10. (canceled) 11.(cancelled)
 12. (canceled)
 13. (canceled)
 14. A heat exchanger accordingto claim 1, whereby the heat transfer fluid flows within independentconduits which are not in direct contact with the gas, liquid or solidwhich is being heated or cooled and that the heat transfer fluid conduitis bonded, fused, glued, brazed, welded or soldered to the surface whichserves as the containment barrier for the gas, liquid or solid which isbeing heated or cooled. 15-58. (canceled)
 59. A heat exchanger accordingto claim 1 wherein the heat transfer fluid conduit or conduits is heldto the surface which serves as the containment barrier for the gas,liquid or solid which is being heated or cooled by means of clamps,springs, wires, natural shape of the conduit or some other formmechanical fixing and a layer of a soft, thermally conductive materialsuch as conductive grease, fluid, conductive wool, fibrous conductivemat or a mixture thereof is provided between the transfer fluid conduitand the surface which serves as the containment barrier for the gas,liquid or solid which is being heated or cooled.
 60. A heat exchangeraccording to claim 1 wherein the conduit for the heat transfer fluid ismounted on an expansion plate to permit independent movement of the heattransfer conduit in relation to the containment barrier for the gas,liquid or solid which is being heated or cooled.
 61. A heat exchangeraccording to claim 1 which uses a variable area heat transfer surface.62. A heat exchanger according to claim 1 in which the residence time ofthe heat transfer fluid is less than 6 seconds.
 63. A heat transfersystem for the transfer of heat between a process fluid and a heattransfer fluid across a heat transfer surface comprising a heat transferconduit for passage of the heat transfer fluid attached to an expansionplate said expansion plate being in contact with the heat transfersurface said expansion plate enabling independent movement of the heattransfer conduit and the heat transfer surface.
 64. A heat transfersystem according to claim 63 wherein the heat transfer fluid isdelivered in at least five heat transfer conduits each having a crosssectional area for the flow path of less than 2000 square millimetreswherein the linear velocity of the heat transfer fluid through the heattransfer conduits is from 0.5 to 20 m·s⁻¹ and adapted so that thetemperature of the heat transfer fluid changes by at least 1° C. whenthey system is operating at full design load.
 65. A heat transfer systemaccording to claim 63 whereby the heat transfer fluid flows withinindependent conduits which are not in direct contact with the gas,liquid or solid which is being heated or cooled and that the heattransfer fluid conduit is bonded, fused, glued, brazed, welded orsoldered to the surface which serves as the containment barrier for thegas, liquid or solid which is being heated or cooled.
 66. A heattransfer system according to claim 63 where the heat transfer fluidflows within independent conduits which are not in direct contact withthe gas, liquid or solid which is being heated or cooled and the heattransfer fluid conduit is held to the surface which serves as thecontainment barrier for the gas, liquid or solid which is being heatedor cooled by means of clamps, springs, wires, natural shape of theconduit or some other form mechanical fixing and the gap between theheat transfer fluid conduit and the surface which serves as thecontainment barrier for the gas, liquid or solid which is being heatedor cooled is filled by means of a soft, thermally conductive materialsuch as conductive grease, fluid, conductive wool, fibrous conductivemat or a composite of several of these materials.